Merge unpacking process into single step

[ehem]

kdztools handles unpacking of KDZ files as a two step process. First, breaking the KDZ format into separate files; second, unpacking the inner DZ file. This matches reasonably to the format, but it appears newer KDZ files are placing extra data between the inner file headers and the start of the inner files.

See commit ehem/kdztools@f34bf672f7f2e3ef82f0c5d4a48692ddd15f3616 which attempts to save off this data for later review (during my kdztools work I'd noticed the potential for extra data and added an error for this situation occurring).